Energy saving is all the rage, and here is our small con¬ tribution: how much (or rather how lit¬ tle) cur¬ rent do we need to light an LED? Experi¬ ments with super-bright 1 W green LED showed that even one microamp was enough to get some visible light from the device.
Rootling in the junk box produced a 0.47 F memory back-up capacitor with a maximum working voltage of 5.5 V.
How long could this power the green LED? In other words, if discharged at one microamp, how long would the voltage take to drop by 1 V? A quick calculation gave the answer as 470 000 seconds, or about five days.
Not too bad: if we use the capacitor for energy storage in a solar-powered lamp we can probably allow a couple more microamps of current and still have the lamp on throughout the night and day. All we need to add is a suit¬ able solar panel. The figure shows the circuit diagram of our (in every sense) green solar lamp.
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